This invention relates to dot matrix line printers and, more particularly, to a mechanism that reduces vibration and backlash in a dot matrix line printer.
Various types of dot matrix line printers have been proposed and are in use. In general, dot matrix line printers include a print head comprising a plurality of dot printing mechanisms, each including a dot-forming element. The dot forming elements are located along a line that lies orthogonal to the direction of paper movement through the printer. Since paper movement is normally vertical, the dot forming elements usually lie along a horizontal line. Located on the side of the paper remote from the dot forming elements is a platen and located between the dot forming elements and the paper is a ribbon. The dot forming elements are spaced a discrete distance along a carriage. To allow dots to be printed closer than this discrete distance, the dot forming elements are shuttled back and forth during printing. As the dot forming elements are shuttled, they are actuated to create dots along the print line defined by the dot forming elements. The paper is incremented forwardly after each dot row is printed. A series of dot rows creates a row of characters or a graphical image.
The dot forming elements of contemporary dot matrix line printers are small anvils located on one end of an electromagnetically actuated hammer. The hammers are normally held in a retracted position by magnetic force. Release is created by the application of a pulse to an electromagnetic coil that produces a magnetic field that counteracts the retracting field. The dot forming element, hammer, retracting magnet, release coil, and related elements form a dot forming mechanism. The dot forming mechanisms may be grouped in sets or modules and the modules mounted on a carriage. See, for example, U.S. Pat. No. 4,351,235, titled Dot Printing Mechanism for Dot Matrix Line Printers.
Shuttling of the dot printing elements back and forth is accomplished by translating the carriage. In one type of line printer, the carriage is reciprocally mounted on a stationary frame by a suitable support mechanism, such as a linear bearing. The carriage shuttle motion is a consequence of a shuttle mechanism such as a crank driving either a connecting rod or a Scotch yoke. One proposed shuttle mechanism for supplying motion to the shuttle comprises a weight unbalanced mechanism that utilizes motor rotated unbalancing weights attached to a supported carriage. The rotation of the unbalancing weights produces a vibration that causes reciprocating carriage movement. Although the unbalanced weight mechanism provides adequate translational movement, there are disadvantages to the unbalanced weight configuration. The main disadvantage with this type of shuttle mechanism is the difficulty of maintaining a stable carriage versus time relationship in the presence of external forces, such as an external bump being applied to the printer, and the difficulty of keeping the print line (the starting point for each line of dots that form the characters) from drifting. If the carriage displacement versus time changes when subject to external forces, the carriage undergoes undesirable motions that result in the misplacing of printed dots. Additionally, if the print line drifts during the operation of the printer, the location of printed dots may be misplaced. Misplacing of printed dots results in imperfectly formed characters, which is unacceptable in modern printers.
In the past, various types of dot matrix line print carriage shuttle systems for maintaining a stable carriage versus time relationship have been proposed. One such proposal is to use the unbalanced weight mechanism in conjunction with a stabilizer device. The stabilizer device utilizes a mechanism, also referred in the art as a Scotch yoke, comprising a rotating crank having an eccentric cam disk riding in a straight slot, to provide a rigid reference point between the carriage and the stationary frame. The stabilizer device also augments translational movement on the carriage to the extent that the counterweights are not perfectly balanced with the carriage.
In this particular configuration, the rotating crank is coupled to the reciprocating carriage and the straight slot is attached to the stationary frame. To eliminate sliding friction between the rotating shaft and the engagement wall of the straight slot, the rotating crank often includes a ball bearing attached to its outer end. Because the bearing cannot contact both slides of the slot at the same time, the slot is made wider than the bearing. This results in undesirable backlash in the mechanism producing unwanted vibration and inaccuracies in the translation position of the carriage. Dot matrix line printers, particularly high speed dot matrix line printers, require precision positioning of the printer head at the time the dot-forming elements are actuated by their related actuating mechanisms. Vibration and backlash reduce the precision with which the print head can be positioned. As print head positioning precision drops, dot misregistration increases. As a result, printed characters and images are distorted and/or blurred. Distorted and/or blurred images are, of course, unacceptable in environments where high quality printing is required or desired.
One method has been proposed to address the unwanted vibration and backlash in the prior stabilizer devices. Specifically, the use of two bearings mounted next to one another on the same rotating shaft has been proposed. The two bearings engage two parallel rails of the stabilizer device that are offset to accommodate the width of the bearing. Typically, the sides of the rails are equipped with rubber springs to preload the bearings to inhibit unwanted backlash. Although the preloaded double bearing functions well to inhibit backlash, there are deficiencies in this configuration. In moving carriage dot matrix printers, the resonance frequency (FR) of the moving carriage mechanism on its support structure must be greater than the operating shuttle frequency (FO) of the moving carriage mechanism on its support structure to provide stability and inhibit vibration in the printer system. To provide a resonance frequency (FR) that meets this requirement, the spring factors of each of the rubber springs (K1, K2) must be significantly large. For a high operating shuttle frequency (FO), if the spring factors of each of the rubber springs (K1, K2) are large enough to satisfy the resonance frequency (FR) requirement, the preload overloads the bearings, resulting in unnecessary mechanical wear. Using bearings that are capable of handling the amount of preload required to provide a resonance frequency (FR) greater than the operating shuttle frequency (FO) would be cost-prohibitive in typical dot matrix line printers.
Therefore, there is a need in the printing industry for a stabilizer device that can provide a rigid reference point that prevents the print line from drifting while eliminating backlash and providing a resonance frequency (FR) that is greater than the operating shuttle frequency (FO) to enhance system stability and the reduction of vibration.
In accordance with the present invention, a preload stabilizer mechanism is provided in a dot matrix line printer having a carriage reciprocated by a shuttling mechanism to address the deficiencies in the prior art. More specifically, the preloaded stabilizer mechanism is provided to: (1) establish a rigid reference point that prevents the print line of the dot matrix line printer from drifting; (2) eliminate backlash; and, (3) create a resonance frequency of the moving carriage mechanism on its support structure that is greater than the operating shuttle frequency of the moving carriage mechanism on its support structure. As a result, the preloaded stabilizer mechanism enhances printer system stability and reduces printer system vibration.
In accordance to an aspect of the present invention, the preload stabilizer comprises a bracket member connectable to a support frame of the printer. The bracket member has two ends. The preload stabilizer mechanism further comprises a spring plate that has a coupled end and a free end. The spring plate is attached to one end of the bracket member to form a slot. The stabilizer mechanism is adapted to receive an offset shaft coupled to the shuttle mechanism within the slot. The spring plate is capable of imposing a preload force against the offset shaft when the offset shaft is within the slot.
In accordance with aspect of this invention, the stabilizer mechanism augments the reciprocating motion to the carriage when the shuttle mechanism is unbalanced.
In accordance with a further aspect of this invention, the stabilizer mechanism prohibits undesirable motions in the carriage as long as the force generated by the unbalanced carriage is less than the force imposed by the spring plate against the offset shaft.
In accordance with yet another aspect of this invention, the stabilizer mechanism provides the printer with a resonance frequency greater than it""s operational frequency.
In one embodiment of the present invention, a printer is provided that comprises a support frame and a carriage reciprocally mounted on the support frame. The carriage has multiple print elements disposed on the carriage. A shuttle mechanism is further coupled to the carriage. The shuttle mechanism causes the carriage to reciprocate on the support frame. The printer further comprises a stabilizer mechanism mounted on the support frame. The stabilizer mechanism includes a bracket member and a spring plate. The spring plate is attached to the bracket member at one end to form a slot. The stabilizer mechanism receives an offset shaft coupled to the shuttle mechanism within the slot. The spring plate imposing a preload force against the offset shaft.